httpclient_test: Increase test_destructor_log timeout

This test has recently become flaky on windows CI, and before
investigating further, see if it's just because the CI machines are
overloaded and subprocesses are slower on windows.

tornado/test/httpclient_test.py

@@ -853,7 +853,7 @@ class SyncHTTPClientSubprocessTest(unittest.TestCase):
             stdout=subprocess.PIPE,
             stderr=subprocess.STDOUT,
             check=True,
-            timeout=5,
+            timeout=15,
         )
         if proc.stdout:
             print("STDOUT:")